1. Field of the Invention
The present invention relates generally to ion mobility sensor systems. Specifically, one aspect of the present invention relates to an extraction sampler and an ion mobility sensor system that uses such an extraction sampler. A second aspect of the present invention relates to an ion mobility sensor system that includes an interface which can be applied between ion mobility spectrometry (IMS) and differential mobility spectrometry (DMS) technology.
2. Discussion of Related Art
Contaminated groundwater and its associated vapor are a major concern due to the persistence of certain pollutants such as dense non-aqueous phase liquids (DNAPLs). Chlorinated hydrocarbons constitute the major portion of DNAPLs and, as such, must be monitored closely. Long-term monitoring (LTM) of these pollutants is needed not only because of their potential hazard, but also due to the reality that complete cleanup of significant DNAPL source zones has not been, and most likely will not be, possible.
Current state-of-the-art technologies for analysis of water contaminants include portable gas chromatography (GC), optical fiber, and membrane mass spectrometry (MS). Most LTM approaches usually involve the installation and maintenance of monitoring wells, labor intensive sampling, and costly laboratory analysis. These technologies are complex, large, time-consuming, require substantial utilities (power and vacuum), and have high associated costs.
For example, in performing membrane mass spectrometry, a hollow membrane is used, such that the outside surface of the membrane is in contact with a sampling substance and the internal chamber of the hollow membrane is connected to a vacuum (often called a “flow-over”). The hollow membrane is used as an inlet for providing sample gases to a mass spectrometer, which then performs mass spectrometry. This “flow-over,” however, is difficult to implement due to conflicting requirements related to the use of a carrier gas. For example, if a carrier gas is used inside this hollow membrane, it can dilute the sample concentration due to the effect of the vacuum in the mass spectrometer. However, if no carrier gas is used the slow diffusion of chemicals from the sample substance along the length of the hollow membrane reduces the gradient of permeate concentrations across the membrane wall and consequently reduces permeability.
The advantages of using IMS technology are numerous, and include high sensitivity, fast response, and low cost. High sensitivity can be attributed to high electron and proton affinities of certain chemicals, as well as larger available sample sizes resulting from configurations which allow for operation in atmospheric pressure.
For example, contra-band drugs have high proton affinities and explosives have high electron affinities. Some chemicals, such as chemical warfare agents and chlorinated hydrocarbons, have both high electron affinities and proton affinities. When these chemicals enter an ionization region of an IMS spectrometer, they will preferentially obtain charge from reactant ions, forming their own characteristic ions in both negative and positive polarities, leading to high sensitivity for IMS technology. Fast response comes from the fact that ions drifting in an IMS drift cell are driven by a constant external field, which results in a fast response time, typically 5-50 milliseconds.
Another intriguing feature is that IMS is generally operated in ambient atmospheric pressure, thus alleviating the problems associated with the vacuum pressurization described with mass spectrometry. Such a feature generates many advantages, including allowing for the use of carrier gases in sampling and separation, as well as providing reliability in robust environments and inexpensive operation compared with mass spectrometry and GC.
However, certain debilitating limits also exist for such IMS spectrometers. Poor resolution is one of them, resulting in cross-sensing and false alarms. Mixture of analytes and complexity of drifting air can also mislead both identification and quantification of analytes.
Despite these limits, IMS technology has been used in many common analytical detection applications. Presently, a large number of IMS sensors are used by the US Army for detecting trace chemical warfare agents. Additionally, more than 10,000 explosive trace detectors, mostly using IMS, have been deployed by the Transportation Security Agency in U.S. airports for the interrogation of carry-on baggage. Many IMS-based detectors are being used for these and other homeland security applications, such as trace detection of drugs and other contra-band.
Although IMS technology has been used for detection of explosives, chemical warfare agents, and other contraband, current handheld IMS systems cannot be used for monitoring chlorinated hydrocarbons in groundwater due to limited resolution. Similarly, a stand-alone differential ion mobility spectrometry (DMS) spectrometer cannot adequately monitor chlorinated hydrocarbons because ions generated from unknown chemicals in groundwater alter the positions and intensities of chlorinated hydrocarbon ion peaks. A combination of an IMS spectrometer and a DMS spectrometer can achieve feasible monitoring of chlorinated hydrocarbons.